A typical indirect reduction of a hydroxyl group of compounds having a partial structure of allyl alcohol has been reported in Proc. Natl. Acad. Sci. USA 74, 4007, (1997), where the hydroxyl group is brominated before the reduction by NaBH.sub.4.
A direct reduction using NaBH.sub.3 CN/ZnI.sub.2 has been reported in J. Org. Chem. 51, 3038 (1986). This method, however, lacks selectivity and involves the reduction of groups other than the desired allylic hydroxyl group and tends to induce allyllic rearrangement, which leads to a poor yield of the product.
Another method which comprises the use of LiAlH.sub.4 /AlCl.sub.3 has been reported in Tetrahedron Letters, 2447 (1976). It is generally known that the method accompanied by reduction of ester groups.
Thus, the existing methods are not suited for the selective reduction of allyl alcohols because they involve side reactions such as the allylic rearrangement and the reduction of undesirable substituents such as ester groups, which are mainly caused by insufficient selectivity. Accordingly, it was difficult to reduce specifically an allylic hydroxyl group of an allyl alcohol-portion to give a desired product in high yield. In addition, an isomer produced by the allylic rearrangement is hardly separable from the desired unsaturated compound even by a chromatographic process, which also lowers the yield.
Accordingly, it has been demanded to establish a method capable of reducing specifically a hydroxyl group of allylic alcohol, avoiding undesirable allyl rearrangement, for an industrial production of the desired product.